1. Field of the Invention
This invention relates to the field of dot matrix printing, with particular reference to creating dot matrix images corresponding to CCITT standard facsimile signals.
2. Prior Art
In recent years, facsimile machines (telecopiers) have become very popular for transmitting graphic information from one place to another. These machines are used to transmit images over the public switched telephone network and are popularly called FAX machines. A FAX transmission can be made as simply as a telephone call, using a FAX machine at each end of the line instead of a telephone instrument. Within each FAX machine are means for producing electrical signals representative of an image to be transmitted, and for coupling these signals to a telephone system, either public or private. Also within each machine is a receiver for receiving FAX signals and for reconstructing an original image in printed form. The present invention is primarily concerned with the printing of images reconstructed from received FAX signals. While the invention has particular application to printing FAX images, and will be described in that context, it will be apparent to those skilled in the art that there are also other applications for the invention as defined by the claims appended hereto.
In order to facilitate FAX communications between large numbers of users, an international organization, The International Telephone and Telegraph Consultative Committee (CCITT), has defined certain standards to be utilized by users of FAX to communicate with other users. These standards are intended to permit communication between machines made by different manufacturers and located in different countries. The CCITT has adopted compatibility standards covering the transmission and reception of FAX signals according to four speed/quality groups: Groups 1, 2, 3, and 4. The vast majority of FAX signals today are sent and received according to the Group 3 compatibility standard. This standard is known as CCITT Recommendation T.4. It was last amended in 1984. Groups 1 and 2 are earlier standards and are relatively slow, while group 4 requires a special high quality telephone line and has not yet gained popularity. The present invention is therefore described in connection with the Group 3 standard. It should be understood, however, that its applicability is not necessarily limited to Group 3 communications.
FAX transmission starts by scanning an original page in raster fashion. The variations of print density on the page are converted into a digital video signal which is subsequently coded for efficient transmission. This coded signal is modulated on an audio carrier and sent over a telephone network. At a remote terminal the signal is received, demodulated, decoded and used to cause a printing device to print an image corresponding to the original.
There are two modes of transmission defined by the CCITT Group 3 standard, standard mode and fine mode. The fine mode raster has a resolution of 7.7 lines per mm (195.6 lines per inch) vertically, with each line including 1728 picture elements ("pels") along a standard 215 mm scan line, i.e., 204 pels per inch horizontally. Each fine mode pel therefore represents a small rectangular area about 5 mils square (1 mil=0.001 inch). The standard mode raster has half the vertical resolution of the fine mode (3.85 lines per mm), but the same horizontal resolution (i.e., 97.8V.times.204H picture elements per inch). A standard mode pel is thus about 10 mils high by about 5 mils wide. According to the CCITT Recommendation T.4, the nominal pel sizes as defined should be maintained within plus or minus one percent.
The printing mechanism of most prior art FAX receivers uses a thermal printhead which is essentially a linear array of resistors, each of which produces heat in response to received electrical signals. By passing thermally sensitive paper over the printhead while the received signals are causing the print elements of the printhead to be heated, dark spots corresponding to the dark picture elements of the original copy are formed. The totality of the dark spots formed during the transmission of a FAX image is intended to be a copy of the original image.
An important disadvantage of the system as described above is the necessity to use special thermally sensitive paper on which the received image is formed. Not only is thermally sensitive paper expensive, but its handling and surface characteristics are such that most people find it to be undesirable. It is therefore an object of the present invention to provide a FAX machine which will receive CCITT standard FAX signals, but will print an image on plain paper rather than on thermally sensitive paper as used in the prior art.
There are some FAX machines presently available which include printers that print on plain paper. These machines, however, utilize xerographic or thermal transfer processes and are relatively expensive.
Ink jet processes have been considered by some in the past for printing FAX images but, generally speaking, ink jet printing has been thought to be impractical because it is typically unreliable and also slow. It is therefore a further object of the present invention to provide a reliable FAX system which can produce images as rapidly as the thermal systems now in use.